In England and Wales there has been a large decline in the birth prevalence of neural tube defects (NTDs) from the early 1970s (reported rates of about 3.2 per 1000 births) to the present (0.1 per 1000 births in 1997). The reported number of terminations of NTD pregnancies increased from 0.02 per 1000 in 1970 to 0.66 per 1000 in 1997, much too small an increase to explain the decline in NTD births. Some underreporting of NTD terminations is recognised. We estimated its extent using 1976-80 data on reported central nervous system (CNS) defect terminations and NTD births in an analysis in which the true total number of NTD pregnancies during this short period was assumed to have either remained constant or else to have been changing by a constant amount per year. The estimate was that 56% of NTD terminations were not reported as such and this fitted the data well. In 1997 the estimated birth prevalence of NTDs was 0.14 per 1000 births, a fall of 96% since 1970. This 96% was apportioned as 40% due to antenatal screening and termination of pregnancy and 56% due to a decline in incidence. Over the period 1970-97 there was an increase in dietary folate, and this will have at least in part caused the decline in incidence of NTDs.